This is an Appendix to our series of articles on Standards.

The FEC packets are created by combining the payload in the media packets using the XOR logical operator. The XOR operator is used because applying it once reverses all the bits in the source. A second application reverses them back to the original state with no loss of data.

The FEC is created by combining media packet 1 with media packet 2 using the XOR technique. Then the result is combined with packet 3 and so on until the last media packet. That final result is used as the FEC.

At the receiving client, the FEC is combined with all the incoming media packets in the same way. If any single packet was dropped, the result will be the content of that packet. The sequence number on the media packets should be monitored to indicate if a recoverable dropout has occurred. Recovery is only possible if ALL of the FEC packets have arrived intact.